Mia has been diagnosed with late infantile NCL (neuronal ceroid lipofuscinosis), which is one form of a condition generically described as Batten disease. The disease is caused by a genetic mutation in one of the 20,000 genes in the body, identified in 1998 to be the CLN2 gene. Less than one in 100,000 people suffer from this CLN2 mutation.

This rare genetic disease causes a deficiency of an enzyme called tripeptidyl peptidase 1 or TPP1. In the absence of TPP1, lipids (fats) that are normally metabolized by this enzyme accumulate in many organs. Buildup of these materials in the cells of the brain and nervous system impair cellular function and cause progressive neurodegeneration and loss of cognitive, motor, and visual functions. In late infantile NCL the onset of symptoms is typically between ages 2 and 4 with an average age of diagnosis of 4 years. The disease progression is rapid. Children typically present initially with seizures and/or vision impairment, followed by ataxia (lack of co-ordination), myoclonus (involuntary muscle twitching), impaired speech, cognitive impairment, and developmental regression. During the later stages of the disease, feeding and tending to everyday needs become very difficult.

Late infantile NCL is inherited from the recessive gene of both parents. In Mia’s case both her parents are carriers of the gene mutation CLN2. Carriers are physically entirely normal, because their normal copy of the gene overrides the potentially harmful effect of the recessive copy with the mistake, but carriers have the potential of passing this faulty gene on to their offspring as a purely random event. The risk of Mia's siblings (she has one younger brother Toby) having the condition is one in four (25%). Recent test results confirm Toby has normal TPP1 enzyme activity, so is NOT affected by CLN2.

Children with late infantile NCL are born healthy and develop normally for the first few years of life. Towards the end of the second year, developmental progress may start to slow down. Some children are slow to talk. The first definite sign of the disease is usually epilepsy. Seizures may be drops, vacant spells or motor seizures with violent jerking of the limbs and loss of consciousness. Seizures may be controlled by medicines for several months but always recur, becoming difficult to control. Children tend to become unsteady on their feet with frequent falls and gradually skills such as walking, playing and speech are lost. Children become less able, and increasingly dependent. By 4-5 years the children usually have myoclonic jerks of their limbs and head nods. They may have difficulties sleeping and become distressed around this time, often for no obvious reason. Vision is gradually lost. By the age of 6 years, most will be completely dependent on families and carers for all of their daily needs. They may need a feeding tube and their arms and legs may become stiff. Some children get frequent chest infections. Death usually occurs between the ages of 6 and 12 years (but occasionally later).

Because pediatric neurodegenerative diseases are so rare, children are sometimes misdiagnosed. Parents often have other children before they realise their child in fact has Batten disease. Since the disease is genetic, this means that some families have more than one child with Batten.